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Technical Information

The nature of the highly pathogenic H5N1 avian influenza virus is complicated. There are many types and subtypes of avian influenza (AI) viruses.

Wild birds are a natural reservoir of AI viruses, most of which are low pathogenic. It is unusual for wild birds to suffer disease from AI viruses. CDFW regularly monitors for this and other bird diseases. Avian influenza viruses, other than the high pathogenic H5N1 type, are constantly present in California's wild bird populations, and always have been.

Scientists categorize avian influenza viruses by the structure of the pathogen itself and then by assigning types and subtypes. There are three types of influenza viruses: A, B, and C. The highly pathogenic H5N1 influenza virus of concern is a Type A virus. All the influenza viruses known to have caused human pandemics throughout history were Type A influenza viruses which is why this one is of particular concern.

Influenza viruses are further classified based on the characteristics of two surface proteins known as hemagglutinin (H) and neuraminidase (N). There are 16 different H subtypes and 9 N subtypes, resulting in a total of 144 different possible combinations of H and N subtypes of influenza A viruses.

The H and N designations do not necessarily identify the pathogenicity of the virus. Within each subtype, genetic sequences determine the pathogenicity of the subtype to an infected bird. There are H5N1 avian influenza viruses that are low pathogenic. In order to be defined as highly pathogenic, the avian influenza virus must kill 75 percent of domestic chickens infected. Anything less than 75 percent is defined as low pathogenic.

It is very possible that a LPAI H5N1 virus in North American birds will be identified that does not cause disease in poultry, their wild birds hosts, or humans. Birds may harbor and shed the virus without getting sick. So, once identified as an H5N1 subtype, further testing must occur to determine the pathogenicity of the virus, either HPAI or LPAI.

What makes a chicken sick may not make another bird or animal species sick. The highly pathogenic H5N1 avian influenza virus makes both chickens and humans sick.